I noticed that when you set the parking brakes on these airliners, the brakes are placed in use at almost their maximum capacity. You can see that with the brake pressure gauge on the Airbus 320, 30, 40, 80 series cockpit - it just shoots all the way up with parking brakes set.

I believe the behaviour is very similar with Boeings as well.

My question is: why? Surely you don't need that much brake pressure to keep the aircraft at a stand still. I do know that very often the parking brake is disengaged when the chocks are in, but still !

I'm not too spun up on the workings of large pax transport but in all of the aircraft that I have flown (everything from small prop jobs, business jets, to heavy jet transports) the parking brake is not a separate brake system like most of us are used to in a car.

Every time we would want to apply the parking brake, we would fully depress the brake pedals (the tips of the rudder pedals) and then activate the parking brake which held the normal wheel brakes on while we desired to keep the aircraft stationary. The parking brake 'catch' would not engage until the brakes were fully depressed.

This might help to explain why you see the brake px increase to max, but I'm still interested in the design aspect myself! Hopefully there are some engineers on the forums.

as said above the parking brake is just the very same brakes as the normal brakes, you just lock the hydraulic pressure inside... so when 3000psi do the job not worse than let´s say 1000psi, why would you want to reduce pressure for the parking brake, when 3000psi are available anyways?

most important is that the extra pressure is to compensate internal and external leakage. over a certain amount of time you might lose some % of pressure through the valves or the brake cylinders. if you have some extra pressure this is irrelevant (almost).

something interesting: many big aircraft brakes have two hydraulic systems powering them, or at least two separate circuits in the brake. on the A340 (don´t know about other A or B) you never power both. if you do so you will compress the brake so much that the cylinders won´t retract anymore... so basically on these brakes you never apply more than 50% of the possible brake force

Quoting AF1624 (Thread starter):My question is: why? Surely you don't need that much brake pressure to keep the aircraft at a stand still.

It's easy. It doesn't hurt the brakes or wheels and it doesn't require any metering...anything different would require some other regulated component to drop hydraulic pressure down to desired brake pressure.

When it's no longer easier (i.e. electric brakes) they don't bother...a 787 with parking brakes set only has the brakes at 30% power.

Quoting AF1624 (Thread starter):
I noticed that when you set the parking brakes on these airliners, the brakes are placed in use at almost their maximum capacity. You can see that with the brake pressure gauge on the Airbus 320, 30, 40, 80 series cockpit - it just shoots all the way up with parking brakes set

On Airbus FBW aircraft the pressure that is indicated on the triple indicator is the brake accumulator pressure on the top, and the alternate brake hydraulic pressure. During normal brake pedal use, the brake pressure gauge is supposed to read zero.

The park brake on the Airbus FBW aircraft uses the alternate brake system, it is a backup to the normal system. The gauges for each system are used during emergency procedure where normal braking is not available. The PM calls out to the PF the pressure, with the idea of trying to exert a constant 1000 psi into the brakes. The accumulator pressure, which is indicated at the top of the triple indicator maybe the only hydraulic pressure available.

We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar

Something else to consider. The Parking Brakes are also used for Engine runs and Hi Power Eng runs. When I do a after eng change run ( pre-Flight Check ), we set the Parking brakes with a certain amount of fuel to perform the checkout. The fuel load changes from acft to acft. The 757 at takeoff power is quite a handfull if too light, and I have seen a 757 drag the wheels on a wet runway. The one Aircraft that has to have the brake Pedals pushed and not use the Parking brakes is the A320. The Parking brakes are not using the full brake accumulator pressure, but a lower metered Pressure, and in some manuals you need a special Big Chocks to put on the mains, when doing a high power run in a A320. The most impressive high power run that had me worried, was a MD-11 & 767 that Pratt had a special plug for the EEC that took the P&W 4000 to 110%. On the wing eng we had to take the other side eng to 80% N1 to keep the plane from going round. I'll never forget those runs.
Just some info that might help

On most types....Setting the Parking brakes is like deppressing the Brakes & locking it in that position.....If hydraulics is switched off as in long halts.....it could require reapplication with Hyd pr to compensate for any leaks.

Quoting AF1624 (Reply 10):Also interesting to know that the 787 has electric brakes!

There was considerable skepticism about those (on a.net and elsewhere) back in the day. During early build there was also press about having to revise the brake software, although I can't find any evidence that that actually led to any delays. However, I'd say they're fantastic. They don't bleed down, so parking brakes are actually parking brakes, they eliminate a ton of hydraulic components and tubes, they don't leak, they're much easier to control, and you can deactivate individual actuators (as opposed to entire brakes) so you've got a lot more MEL options. I'm a fan.

The only big "gotcha" I've seen on them so far is in heavy maintenance...if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Quoting boeing767mech (Reply 11):I talked to a mechanic here at my place of employment he said the Fokker F-27 had pnuematic brakes.

The F27 had no hydraulic system, it was all air driven.
The accumulators were charged by an engine driven pheumatic pump, but it took a while. Gulf Air used to operate the F27 BAH-DHA-BAH , with flight times of around 10 mins each way. We had to recharge the air accumulators from a bottle every BAH turnround.

Quoting HAWK21M (Reply 9):On most types....Setting the Parking brakes is like deppressing the Brakes & locking it in that position.....If hydraulics is switched off as in long halts.....it could require reapplication with Hyd pr to compensate for any leaks.

Thats the Boeing way. Airbus is different, you just turn the park brake from off to on. And Airbus brakes are still on the next morning!!

Quoting tdscanuck (Reply 12):There was considerable skepticism about those (on a.net and elsewhere) back in the day. During early build there was also press about having to revise the brake software, although I can't find any evidence that that actually led to any delays. However, I'd say they're fantastic. They don't bleed down, so parking brakes are actually parking brakes, they eliminate a ton of hydraulic components and tubes, they don't leak, they're much easier to control, and you can deactivate individual actuators (as opposed to entire brakes) so you've got a lot more MEL options. I'm a fan.

The only big "gotcha" I've seen on them so far is in heavy maintenance...if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Tom.

Kinda like jacking a 737 without pulling the correct breakers, you end up with a helluva mess if you have to swing the gear. LOL.

On the 737NG the brakes are depressed (the same way you'd push them to stop the plane when it's taxiing) then the lever is pulled. This lever will cause the brake pressure to not release. If the hydraulic systems are shut off (ie. when the APU is turned off) will the pressure stay in the brakes to hold the plane? I would think the parking brake is just simply a valve which closes to keep the fluid pressure from escaping.

Quoting boeing767mech (Reply 11):I talked to a mechanic here at my place of employment he said the Fokker F-27 had pnuematic brakes.

Those Pneumatic brake systems on that type was a tough snag to troubleshoot.

Quoting tdscanuck (Reply 12):if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Normally the precaution prior to Fuselage Jacking.

Quoting Tristarsteve (Reply 14):Thats the Boeing way. Airbus is different, you just turn the park brake from off to on. And Airbus brakes are still on the next morning!!

Quoting tdscanuck (Reply 12):
There was considerable skepticism about those (on a.net and elsewhere) back in the day. During early build there was also press about having to revise the brake software, although I can't find any evidence that that actually led to any delays. However, I'd say they're fantastic. They don't bleed down, so parking brakes are actually parking brakes, they eliminate a ton of hydraulic components and tubes, they don't leak, they're much easier to control, and you can deactivate individual actuators (as opposed to entire brakes) so you've got a lot more MEL options. I'm a fan.

The only big "gotcha" I've seen on them so far is in heavy maintenance...if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Tom.

Tom, what happens if and when the 787 loses all electric power at the gate ?

For example, external power trips off and APU / Engine generators all off.

Quoting horstroad (Reply 2): so when 3000psi do the job not worse than let´s say 1000psi, why would you want to reduce pressure for the parking brake, when 3000psi are available anyways?

Often times you don't want any pressure on them. The Airbus 330 & 340s I used to work specifically called out for breaks release after chocks are set. On short turns, this is necessary to allow adequate break cooling.

Quoting horstroad (Reply 2):most important is that the extra pressure is to compensate internal and external leakage. over a certain amount of time you might lose some % of pressure through the valves or the brake cylinders. if you have some extra pressure this is irrelevant (almost).

Chocks generally solve that concern.

Quoting n901wa (Reply 7):The one Aircraft that has to have the brake Pedals pushed and not use the Parking brakes is the A320.

DHC-8 100, 200 & 300 all work that way as well.

Quoting tdscanuck (Reply 12):The only big "gotcha" I've seen on them so far is in heavy maintenance...if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Any chance they'll ever put a manual release feature for just that contingency. I understand that should be an avoidable encounter, but shit, as they say, happens.

I've done many run-ups on the Dash 8 100 and 300 using the parking brake. Are you saying you have to hold the toe brakes to do a run-up? I do keep my feet on the pedals just to be safe though, 4600 horsepower is something to be respected.

Quoting JAGflyer (Reply 16):On the 737NG the brakes are depressed (the same way you'd push them to stop the plane when it's taxiing) then the lever is pulled. This lever will cause the brake pressure to not release. If the hydraulic systems are shut off (ie. when the APU is turned off) will the pressure stay in the brakes to hold the plane?

Yes. In a proper functioning system the parking brake will hold for a minimum of eight hours with "B" hydraulics off.

Quoting JAGflyer (Reply 16):I would think the parking brake is just simply a valve which closes to keep the fluid pressure from escaping.

On the NG, when the parking brake lever is set, the rudder/brake pedals are mechanically held down which keep the metering valves open and pressure applied to the brakes. At the same time there is a parking brake shutoff valve that closes the return line from the normal antiskid valves, otherwise there would be unacceptable leakage through them.

Quoting Max Q (Reply 18):Tom, what happens if and when the 787 loses all electric power at the gate ?

For example, external power trips off and APU / Engine generators all off.

Does the parking brake remain set using battery power ?

Basically, in the event of an external failure, they'll go to battery. In the event of a battery failure they'll just stay where they were.

The brakes run off the battery bus, so you can use the brakes just by turning the battery on. If the brakes were already set when the power went off, they'd just stay set. The brake actuators don't backdrive; the brakes can't back off without electric power to the motors. The only way you could end up without full braking is if you set the brakes right on arrival then immediately lost power...as the brake stack cooled the clamping force would drop off. As long as you have power (even battery power will do) the brakes will adjust to the cooling for a while after setting the parking brake (something like an hour, I think).

Quoting titanmiller (Reply 19):How long does it take the hydraulic pressure to bleed off if an aircraft sits for an extended period of time with the parking breaks set?

Depends on how well the hydraulics have been taken care of. Typically hours.

Quoting Darksnowynight (Reply 20):Any chance they'll ever put a manual release feature for just that contingency. I understand that should be an avoidable encounter, but shit, as they say, happens.

As a single-step release, I doubt it. That's a lot of work for a rare (and preventable) occurence. If you're really stuck you could just remove the actuators or "hotwire" them by opening the connector at each actuator and applying DC directly to the actuator motors.